External style sheets have many powerful that make them ubiquitous in professional Web sites:
It keeps your website design and content separate.
It's much easier to reuse your CSS code if you have it in a separate file. Instead of typing the same CSS code on every web page you have, simply have many pages refer to a single CSS file with the "link" tag.
You can make drastic changes to your web pages with just a few changes in a single CSS file.
It allows a single style sheet to control the rendering of multiple documents.
This results in a time-savings for the author, a savings of space for the web server, and less download time for the user.
This method can be used in both HTML and XML.

You must have came across some .css issues related with the browser behavior. Some of the styles that works in IE doesn't work in FireFox.
In that situation you need to write conditional code for both browsers.